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How to save money and get poorer (with special reference to the red meat industry)

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By Nigel Amschwand, F.Inst.R

In these difficult times it is very easy to justify taking the low-cost option. However, in many cases, particularly in the red meat industry, the most expensive offer may be the best financial solution, even in the short term – one to two years.

An example of a beef carcass chiller. Photo supplied
An example of a beef carcass chiller. Photo supplied

The biggest differences are using standard air coolers versus custom selected items and the choice of refrigeration systems.

Standard air coolers are produced in a wide range of sizes and widely used in commercial cold stores. They are rated for standard conditions, perhaps a sensible heat ratio (SHR) of 0.85, no frost and a specified temperature difference between the air and refrigerant temperatures. They also come with a fixed air quantity at relatively low external pressure drops. This is not to criticise standard air coolers but to provide a better understanding of them.

Let’s take a look at these items individually.

Sensible heat ratio

The sensible heat ratio is the ratio of the sensible heat to the total heat (sensible + latent). In many applications a SHR of 0.85 is acceptable. In a carcass chiller the latent load, by design, must be kept as small as possible. Any latent load means moisture is extracted from the carcass. More about that later.

The higher the latent heat – therefore the lower the SHR, results in a higher heat transfer coefficient for the air cooler. Wet fins conduct heat better than dry ones. Hence, a cooler having a stated capacity at a SHR of 0.85 will have less capacity when operating at a designed SHR of 0.95.


All air coolers with air-off temperatures close to zero and refrigerant temperatures below zero will accumulate frost. The frost adds an insulating layer to the surface of the fins. An air cooler having a heat transfer coefficient of 36 W/m2.K, will, with 1mm of frost, have 15% less capacity.

Another consideration is the fin spacing. A narrow fin spacing of 6 mm with 1 mm of frost will have a greatly reduced free area and considerably less air flow.

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Air to refrigerant temperature difference

It is important to understand what the specified temperature difference is – as stated in the literature.

The correct method uses the air-on temperature as the room temperature. It is also possible to consider the mean room temperature as the air on temperature. As long as this is known, it can be compensated for. See Diagram 1.

Diagram 1. Supplied - Goedhart BV
Diagram 1. Supplied – Goedhart BV

In this example (Illustration 1), A, where the air-on temperature is taken as the room temperature the LMΔT is 5.77K and the cooler surface (for 40 kW) is 346 m2. Whereas, in B. using the average room temperature, the LMΔT is 7.85K and the cooler surface 255 m2. If you think you are a using a cooler rated as A, but in fact rated as B, your cooler will be 26% undersized.

The temperature difference between the air and the refrigerant is shown on Diagram 2.

This is the log mean temperature difference, excluding the superheat (for direct expansion air coolers) but including most importantly the refrigerant pressure drop in the coil.

From the illustration in Diagram 2, we can see that it is in many cases necessary to custom design air coolers, especially for carcass chilling. The selection must include for the high SHR and the fin spacing should be at least 10 mm to allow for defrosting only between cycles. Each half hour defrost mid-cycle will extend the cooling cycle by up to an hour.

Diagram 2. Supplied - Goedhart BV
Diagram 2. Supplied – Goedhart BV

The air coolers must also be designed for the correct capacity. The cooling rate is dependent on the ability of the product to release its heat. The cooler capacity is not the total product heat divided by the required cooling time, plus the room losses, but must cope with the peak load otherwise the coolers will be too small, and the cooling time not achieved.

The heat extracted from the carcasses is by a combination of air temperature and velocity. The air temperature cannot be too low, otherwise there is a risk of freezing the meat. Therefore, velocity is a critical factor. It is particularly important that the velocity is equal over all the carcasses.

The final aspect is the choice of refrigeration system.

Direct expansion systems need a superheat to control the refrigerant flow. This superheat increases the temperature difference between the air and the refrigerant. As the air temperature is controlled, the refrigerant temperature must be lower. The lower the refrigerant temperature the more moisture is removed from the air.

Direct expansion systems generally cannot maintain a relative humidity above 80-85%. Conversely, flooded refrigeration systems can operate at very small temperature differences and maintain a very high humidity.

So, where is all this leading? Let’s consider a typical pig chiller holding 300, 80kg carcasses – a total of 24 000kgs of produce.

An adequately designed, conventional system with flooded refrigerant (high humidity) and correct air circulation will give a weight loss of 1 – 1.5% during the chilling cycle. Using 1.5% in the calculations, this equals 360kg less coming out of the chiller than what went in. A poorly designed chiller with inadequate air circulation and lower relative humidity would give about 2.5% weight loss.

Experience has shown that using standard coolers, not correctly selected, having inadequate air circulation, fin spacing and not allowing for peak heat load, results in a weight loss of 4% or more. Plus, the cooling time to a deep bone temperature of 7°C or lower will be longer. Four percent equals 960kg – a difference of 600kg from an adequately designed system.

At an estimated wholesale price of R42 per kg this has a value of R25 200. This is every day for each chiller and will cost you R5-million per year and for every year into the future.

This need for custom designed air coolers and flooded refrigeration systems, usually ammonia, is not limited to carcass chilling. Any product that needs high humidity storage conditions will be the same. Controlled atmosphere fruit stores, where the product is held for many months, have experienced poor quality product due to incorrect cooler selection.

The opinions stated are those of the author based on many years designing systems for the meat industry. Anyone having another opinion is welcome to correspond further.

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